1. Field of the Invention
The present invention relates to apparatus and methods for remotely installing vortex-induced vibration (VIV) and drag reduction devices on structures in flowing fluid environments. In another aspect, the present invention relates to apparatus and methods for installing VIV and drag reduction devices on underwater structures using equipment that can be remotely operated from above the surface of the water. In even another aspect, the present invention relates to apparatus and methods for remotely installing VIV and drag reduction devices on structures in an atmospheric environment using equipment that can be operated from the surface of the ground.
2. Description of the Related Art
Whenever a bluff body, such as a cylinder, experiences a current in a flowing fluid environment, it is possible for the body to experience vortex-induced vibrations (VIV). These vibrations are caused by oscillating dynamic forces on the surface which can cause substantial vibrations of the structure, especially if the forcing frequency is at or near a structural natural frequency. The vibrations are largest in the transverse (to flow) direction; however, in-line vibrations can also cause stresses which are sometimes larger than those in the transverse direction.
Drilling for and/or producing hydrocarbons or the like from subterranean deposits which exist under a body of water exposes underwater drilling and production equipment to water currents and the possibility of VIV. Equipment exposed to VIV includes structures ranging from the smaller tubes of a riser system, anchoring tendons, or lateral pipelines to the larger underwater cylinders of the hull of a minispar or spar floating production system (hereinafter “spar”).
Risers are discussed here as a non-exclusive example of an aquatic element subject to VIV. A riser system is used for establishing fluid communication between the surface and the bottom of a water body. The principal purpose of the riser is to provide a fluid flow path between a drilling vessel and a well bore and to guide a drill string to the well bore.
A typical riser system normally consists of one or more fluid-conducting conduits which extend from the surface to a structure (e.g., wellhead) on the bottom of a water body. For example, in the drilling of a submerged well, a drilling riser usually consists of a main conduit through which the drill string is lowered and through which the drilling mud is circulated from the lower end of the drill string back to the surface. In addition to the main conduit, it is conventional to provide auxiliary conduits, e.g., choke and kill lines, etc., which extend parallel to and are carried by the main conduit.
This drilling for and/or producing of hydrocarbons from aquatic, and especially offshore, fields has created many unique engineering challenges. For example, in order to limit the angular deflections of the upper and lower ends of the riser pipe or anchor tendons and to provide required resistance to lateral forces, it is common practice to use apparatus for adding axial tension to the riser pipe string. Further complexities are added when the drilling structure is a floating vessel, as the tensioning apparatus must accommodate considerable heave due to wave action. Still further, the lateral forces due to current drag require some means for resisting them whether the drilling structure is a floating vessel or a platform fixed to the subsurface level.
The magnitude of the stresses on the riser pipe, tendons or spars is generally a function of and increases with the velocity of the water current passing these structures and the length of the structure.
It is noted that even moderate velocity currents in flowing fluid environments acting on linear structures can cause stresses. Such moderate or higher currents are readily encountered when drilling for offshore oil and gas at greater depths in the ocean or in an ocean inlet or near a river mouth.
Drilling in ever deeper water depths requires longer riser pipe strings which because of their increased length and subsequent greater surface area are subject to greater drag forces which must be resisted by more tension. This is believed to occur as the resistance to lateral forces due to the bending stresses in the riser decreases as the depth of the body of water increases.
Accordingly, the adverse effects of drag forces against a riser or other structure caused by strong and shifting currents in these deeper waters increase and set up stresses in the structure which can lead to severe fatigue and/or failure of the structure if left unchecked.
There are generally two kinds of current-induced stresses in flowing fluid environments. The first kind of stress is caused by vortex-induced alternating forces that vibrate the structure (“vortex-induced vibrations”) in a direction perpendicular to the direction of the current. When fluid flows past the structure, vortices are alternately shed from each side of the structure. This produces a fluctuating force on the structure transverse to the current. If the frequency of this harmonic load is near the resonant frequency of the structure, large vibrations transverse to the current can occur. These vibrations can, depending on the stiffness and the strength of the structure and any welds, lead to unacceptably short fatigue lives. In fact, stresses caused by high current conditions in marine environments have been known to cause structures such as risers to break apart and fall to the ocean floor.
The second type of stress is caused by drag forces which push the structure in the direction of the current due to the structure's resistance to fluid flow. The drag forces are amplified by vortex induced vibrations of the structure. For instance, a riser pipe that is vibrating due to vortex shedding will disrupt the flow of water around it more than a stationary riser. This results in more energy transfer from the current to the riser, and hence more drag.
Many types of devices have been developed to reduce vibrations of subsea structures. Some of these devices used to reduce vibrations caused by vortex shedding from subsea structures operate by stabilization of the wake. These methods include use of streamlined fairings, wake splitters and flags.
Streamlined or teardrop shaped, fairings that swivel around a structure have been developed that almost eliminate the shedding of vortices. The major drawbacks to teardrop shaped fairings is the cost of the fairing and the time required to install such fairings. Additionally, the critically required rotation of the fairing around the structure is challenged by long-term operation in the undersea environment. Over time in the harsh marine environment, fairing rotation may either be hindered or stopped altogether. Anon-rotating fairing subjected to a cross-current may result in vortex shedding that induces greater vibration than the bare structure would incur.
Other devices used to reduce vibrations caused by vortex shedding from sub-sea structures operate by modifying the boundary layer of the flow around the structure to prevent the correlation of vortex shedding along the length of the structure. Examples of such devices include sleeve-like devices such as helical strakes, shrouds, fairings and substantially cylindrical sleeves.
Some VIV and drag reduction devices can be installed on risers and similar structures before those structures are deployed underwater. Alternatively, VIV and drag reduction devices can be installed by divers on structures after those structures are deployed underwater.
Use of human divers to install VIV and drag reduction equipment at shallower depths can be cost effective. However, strong currents can also occur at great depths causing VIV and drag of risers and other underwater structures at those greater depths. However, using divers to install VIV and drag reduction equipment at greater depths subjects divers to greater risks and the divers cannot work as long as they can at shallower depths. The fees charged, therefore, by diving contractors are much greater for work at greater depths than for shallower depths. Also, the time required by divers to complete work at greater depths is greater than at shallower depths, both because of the shorter work periods for divers working at great depths and the greater travel time for divers working at greater depths. This greater travel time is caused not only by greater distances between an underwater work site and the water surface, but also by the requirement that divers returning from greater depths ascend slowly to the surface. Slow ascent allows gases, such as nitrogen, dissolved in the diver's blood caused by breathing air at greater depths, to slowly return to a gaseous state without forming bubbles in the diver's blood circulation system. Bubbles formed in the blood of a diver who ascends too rapidly cause the diver to experience the debilitating symptoms of the bends.
Elongated structures in wind in the atmosphere can also encounter VIV and drag, comparable to that encountered in aquatic environments. Likewise, elongated structures with excessive VIV and drag forces that extend far above the ground can be difficult, expensive and dangerous to reach by human workers to install VIV and drag reduction devices.
However, in spite of the above advancements, there still exists a need in the art for apparatus and methods for installing VIV and drag reduction devices on structures in flowing fluid environments.
There is another need in the art for apparatus and methods for installing VIV and drag reduction devices on structures in flowing fluid environments, which do not suffer from the disadvantages of the prior art apparatus and methods.
There is even another need in the art for apparatus and methods for installing VIV and drag reduction equipment on underwater structures without using human divers.
There is still another need in the art for apparatus and methods for installing VIV and drag reduction devices on underwater structures using equipment that can be remotely operated from the surface of the water.
There is yet another need in the art for apparatus and methods for installing VIV and drag reduction devices on above-ground devices using equipment that can be operated from the surface of the ground.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.